1. Field of the Invention
The present invention relates to side-look sonar systems, and more particularly to multibeam side-look sonar systems.
2. Description of Related Art
In a multibeam side-look sonar system, a projecting transducer transported on the side of an underwater carrier vehicle projects a beam of acoustic energy to insonify a narrow target area. The target area reflects acoustic energy back to a receiving transducer also transported on the carrier vehicle. The receiving transducer typically is divided into a plurality of individual transducer elements arranged in a linear array. In response to the acoustic energy reflected from the target area, the transducer elements generate beam signals that effectively provide a spatial sampling of the acoustic energy received along the aperture of the transducer array. A beamforming circuit processes the beam signals generated by the transducer elements to form a beam pattern centered on a particular transducer element in the linear array. The distance separating the center points of adjacent transducer elements causes repetitions of the wave of acoustic energy along the aperture of the array. When the beamforming circuit forms a beam pattern for a particular set of transducer elements, the repetitions caused by the distance between the transducer elements form grating side lobes in the beam pattern. The grating side lobes are formed at angles relative to a main lobe in the beam pattern, in directions in which there is a one wavelength difference in the arrival of the reflected acoustic energy between adjacent transducer elements in the linear array.
Beam patterns formed for transducer elements positioned near the center of the linear array are generally called "center beams," whereas beam patterns formed with respect to transducer elements positioned away from the center of the array are generally called "end beams." For a center beam, the beam pattern includes a main lobe within the target area insonified by the projecting transducer, and grating side lobes that tend to fall outside of the target area. At short range, the grating side lobes for the center beam generally continue to fall outside of, or "straddle," the narrow target area. However, the beam pattern for an end beam may include a grating side lobe that falls within the insonified target area. For short range targets, in particular, one of the grating side lobes for the end beam invariably falls within the target area. Unfortunately, when one of the grating side lobes is insonified, the side lobe signal is added to the main lobe signal blurring the sonar image. The resulting ambiguous beam pattern can seriously compromise the imaging performance of the side-look sonar system. Accordingly, there is a need for a multibeam side-look sonar system for reducing the effects of grating side lobes.